1. Field of the Invention
The present invention relates to the implementation of a microtip cathode of a flat display screen.
2. Discussion of the Related Art
FIG. 1 shows an example of conventional structure of a flat color microtip screen.
Such a microtip screen is essentially formed of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate of which generally forms the screen surface.
Cathode 1 is organized in columns and is formed, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. Microtips 2 are made on a resistive layer 11 deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh and the cathode conductors do not appear on the drawing. Cathode 1 is associated with grid 3 organized in lines. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
This device uses the electric field created between cathode 1 and grid 3 to extract electrons from microtips 2. These electrons are then attracted by phosphor elements 7 of anode 5 if these elements are properly biased. In the case of a color screen such as shown in FIG. 1, anode 5 is provided with alternate strips of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The strips are parallel to the cathode columns and are separated from one another by an insulator 8. Phosphors 7 are deposited on electrodes 9, formed of corresponding strips of a transparent conductive layer such as indium and tin oxide (ITO). The sets of red, green, blue strips are alternately biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed to the phosphor elements 7 facing each of the colors.
In the case of a monochrome screen (not shown), the anode is formed of a plane of phosphor elements of same color or of two sets of alternate strips of phosphor elements of same color.
FIGS. 2A and 2B schematically illustrate the meshing of the cathode conductors of such a microtip screen. FIG. 2A partially shows in top view a microtip cathode and FIG. 2B is a cross-sectional view along line B-B' of FIG. 2A. For clarity, the grid (3, FIG. 1) and the insulating layer between this grid and the resistive layer (11, FIG. 1) have not been shown in FIGS. 2A and 2B.
Several microtips 2, for example, 16, are arranged in each mesh 12 defined by cathode conductors 13. Although a reduced number of meshes has been shown for each pixel 14 defined by the intersection of a column 15 of cathode 1 and of a line of the grid (not shown), it should be noted that the microtips generally are as many as several thousands per screen pixel.
Cathode 1 is generally formed of layers successively deposited on glass substrate 10. A conductive layer 13, for example, made of niobium, is deposited on substrate 10. This layer 13 is etched according to the pattern of columns 15, each column including meshes 12 surrounded with cathode conductors 13. A resistive layer 11 is then deposited on cathode conductors 13. Resistive layer 11, formed, for example, of phosphorous-doped amorphous silicon, has the object of protecting each microtip 2 against a current excess at the starting of a microtip 2. The addition of such a resistive layer 11 aims at homogenizing the electron emission of the microtips 2 of a pixel of cathode 1 and thus at increasing its lifetime. The resistive layer may be etched according to the column pattern and/or opened, at least partially, above the cathode conductors. An insulating layer (not shown), for example in silicon oxide (SiO.sub.2), is deposited on resistive layer 11 to insulate cathode conductors 13 of grid 3 (FIG. 1). A microtip cathode of this type is described, for example, in European patent application n.sup.o 0696045.
If desired, cathode conductors 13 may be deposited on resistive layer 11 which may, as in the preceding case, be a full plate layer or not. A microtip cathode of this type is described, for example, in French patent application n.sup.o 2722913.
A disadvantage of conventional screens is that, along the screen operation, differences in brightness can be observed from one column of the screen to another, which are due, in particular, to a drift in the amount of electrons emitted by the cathode microtip columns for a given luminance reference. This phenomenon which occurs both for color screens and for monochrome screens results in the appearing of overbright columns independently from the image pattern to be displayed.